Bug? or do I something wrong

[Daniel Kozak via Digitalmars-d]

import std.stdio;

void main() {
    auto x = 9223372036854775807L;
    auto x2 = 9223372036854775807L + 1;
    long x3 = -9223372036854775808U;
    //auto x4 = -9223372036854775808L; //Error: signed integer 
overflow

    writeln(x + 1); //-9223372036854775808
    writeln(x2);    //-9223372036854775808
    writeln(x3);    //-9223372036854775808
    //writeln(x4);
}